Figure 1.—Incidence of selected cancers in the United States
by birth cohort and simian virus 40–contaminated vaccine exposure. Superimposed
on the incidence rates in the figure are the best-fitting Poisson models that
were used to describe the data statistically.

Figure 2.—Incidence of ependymoma among children 0 to 4 years
of age in Connecticut, "before" (1950-1954), "during" (1955-1964), and "after"
(1965-1969) the mass immunization of children with simian virus 40–contaminated
poliovirus vaccines. The number of cases during each period is indicated in
parentheses at the top of each bar.

Context.— Poliovirus vaccine contaminated with live simian virus 40 (SV40), a
macaque polyomavirus that is tumorigenic in rodents, was used extensively
in the United States between 1955 and 1963. Simian virus 40 DNA has recently
been detected in several rare human tumors, including ependymomas, osteosarcomas,
and mesotheliomas.

Objective.— To determine the risk of ependymoma, osteosarcoma, and mesothelioma
among Americans who as children received SV40-contaminated poliovirus vaccine.

Design.— Retrospective cohort study using data from the Surveillance, Epidemiology,
and End Results program (1973-1993) and the Connecticut Tumor Registry (1950-1969),
as well as national mortality statistics (1947-1973).

Setting.— United States.

Participants.— Birth cohorts that were likely to have received SV40-contaminated poliovirus
vaccine as infants, born 1956 through 1962 (60811730 person-years of observation);
as children, born 1947 through 1952 (46430953 person-years); or that were
unexposed, born 1964 through 1969 (44959979 person-years).

Main Outcome Measures.— Relative risk (RR) of each cancer among exposed compared with unexposed
birth cohorts.

Results.— Age-specific cancer rates were generally low and were not significantly
elevated in birth cohorts exposed to SV40-contaminated vaccine. Specifically,
compared with the unexposed, the relative risk of ependymoma was not increased
in the cohorts exposed as infants (RR, 1.06; 95% confidence interval [CI],
0.69-1.63), or as children (RR, 0.98; 95% CI, 0.57-1.69) nor did the exposed
have an increased risk of all brain cancers. Osteosarcoma incidence also showed
no relation to exposure as infants (RR, 0.87; 95% CI, 0.71-1.06) or children
(RR, 0.85; 95% CI, 0.59-1.22). Last, mesotheliomas were not significantly
associated with exposure, although the cohorts studied have not yet reached
the age at which these tumors tend to occur.

Conclusions.— After more than 30 years of follow-up, exposure to SV40-contaminated
poliovirus vaccine was not associated with significantly increased rates of
ependymomas and other brain cancers, osteosarcomas, or mesotheliomas in the
United States.

DNA SEQUENCES homologous to simian virus 40 (SV40), a macaque polyomavirus
that can induce cancer in rodents,1,2
were recently detected in several rare human tumors, including ependymomas,3- 5 osteosarcomas,6 and mesotheliomas.7
Tens of millions of Americans were exposed to this virus between 1955 and
1963 as a consequence of adventitious contamination of the early poliovirus
vaccines, produced in Asian macaque kidney cell cultures. By 1961, between
80% to 90% of all US children younger than 20 years had been injected at least
once with formalin-inactivated poliovirus vaccine (IPV) containing SV40.8 Because SV40 is relatively resistant to formalin killing,
the IPV contained variable amounts, commonly low titers, of live SV40. The
oral poliovirus vaccine began mass distribution in the United States in 1963
and was SV40-negative.8

Earlier studies of cancer risk following exposure to SV40-contaminated
vaccines were generally limited by small sample size or short follow-up.9- 15
One exception, a large study in the German Democratic Republic with 22 years
of follow-up, found no significant differences in cancer rates between the
885783 individuals who received SV40-contaminated poliovirus vaccine as infants,
compared with similarly aged individuals born a few years later, who received
only SV40-negative vaccine.16

No epidemiologic studies, however, have evaluated the specific types
of cancers found recently to contain SV40 DNA. In addition, many of the earlier
investigations, including the German study, examined mainly oral poliovirus
vaccine. The impact of the major single-source exposure to SV40 in the United
States, injected IPV,8 has not been adequately
assessed. Both animal and human studies have shown that the route of SV40
exposure is biologically important.8,9
Notably, tumors in animals were all induced by injection and neonates were
particularly susceptible.1,2,17
Therefore, more than 30 years after millions of American infants and children
were immunized with SV40-contaminated poliovirus vaccine, it is now possible
to investigate the long-term carcinogenic effects of parenteral exposure to
SV40 in early life.

Methods

The risk of immunization with SV40-contaminated IPV was determined according
to birth cohort based on published information,8
and was used to define 3 comparison groups: (1) individuals at high risk of
exposure in infancy, born 1956 through 1962; (2) those at high risk of exposure
as children, born 1947 through 1952; and (3) unexposed individuals born a
few years later, 1964 through 1969. Cancer incidence and mortality rates in
these 3 cohorts were then compared on an age-specific basis (as described
below).

Cancer incidence rates in the United States were obtained from the Surveillance,
Epidemiology, and End Results (SEER) program of the National Cancer Institute,
which since 1973 has collected detailed information regarding new cancer cases
diagnosed among residents of 9 representative areas with approximately 10%
of the total US population.18 Additional incidence
data were obtained from the Connecticut Tumor Registry, the only cancer registry
in the United States that was well established prior to 1955. Cancer mortality
data for the entire country were obtained from the National Center for Health
Statistics, and the population and demographic data from the US Bureau of
the Census.

The cancers studied included ependymomas, osteosarcomas, and mesotheliomas,
which have been reported to contain SV40 DNA. In addition, all primary brain
cancers were studied as a group, since it has been suggested that a variety
of brain tumors might contain SV40 DNA.5 For
each birth cohort, we calculated age-specific cancer incidence rates by single
year of age per 100000 person-years at risk. We used Poisson regression to
assess whether the age-specific incidence rates varied according to birth
cohort, and fitted a sequence of models to assess whether the relationship
between the log of the incidence rate and age was best described as uniform,
linear, quadratic, or as a cubic spline with 2 or 3 segments. We used the
likelihood ratio test to determine the best-fit model for age and the significance
of birth cohort. In summary, we optimally controlled for the effects of age
to best assess whether exposure history (determined by year of birth) was
related to the incidence of cancer. For individual age-specific incidence
or mortality figures of special interest, 95% confidence intervals (CIs) were
determined assuming a Poisson distribution.19
Last, time trends in age-specific cancer incidence in Connecticut from 1950
through 1969 were examined for any changes in rates that could be attributed
to SV40-contaminated vaccine exposure.

Results

The observed and fitted (smoothed) age-specific cancer incidence rates
in the SEER catchment area for 1973 through 1993 are presented by birth cohort
in Figure 1. In general, the exposed
groups did not experience elevated rates of cancer, and the likelihood ratio
tests found no significant increases in cancer risk among the cohorts exposed
as infants (60811730 person-years of observation for each cancer studied)
or children (46430953 person-years), compared with the unexposed birth cohort
(44959979 person-years).

Ependymoma incidence rates, based on 200 total cases in the data set,
were similar in each of the 3 comparison groups (Figure 1, A), with observed fluctuations reflecting the small numbers
of this rare tumor at any given age. For example, at age 13 years there were
5 cases (95% CI, 2.15-15.43) compared with 2 cases (95% CI, 0.55-16.53) in
the cohorts that were exposed as infants and unexposed, respectively. Ependymoma
incidence was best fitted using a quadratic model for age, and showed no overall
difference among the cohorts (χ2, 0.19 on 2 df; P=.91). Specifically, incidence in the
cohorts exposed as infants (RR, 1.06; 95% CI, 0.69-1.63) or children (RR,
0.98; 95% CI, 0.57-1.69) was not elevated as compared with the unexposed cohort
(goodness of fit, 70.2 on 77 df ).

Since the SEER program began in 1973, these data could not be used to
study ependymoma incidence in the age group at highest risk, children under
the age of 4 years.20 To address this limitation,
we studied time trends in incidence among children 0 to 4 years of age in
Connecticut, from 1950 to 1969 (Figure 2).
Ependymoma incidence in this age group (based on 22 cases and 5036496 person-years
of observation) was actually higher during the period 1950 through 1954, just
prior to the mass immunization program, than in 1960 through 1964, when the
greatest effect of SV40 exposure on ependymoma incidence would be expected;
ie, as a result of exposures during both 1954 through 1959 and 1960 through
1963. Similar data for individuals 5 to 9 and 10 to 14 years of age in Connecticut
also showed no relation between ependymoma incidence and the period of vaccine
contamination.

Brain cancer incidence (Figure 1,
B), was best fitted using a 2-segment spline function for age (goodness of
fit, 62.71 on 75 df ). These tumors were relatively
common (4162 total cases) and the variation according to birth cohort was
statistically significant (χ2, 10.89 on 2 df; P=.004). However, compared with the unexposed cohort, incidence
was incrementally lower in the cohorts exposed to SV40-contaminated vaccine
as infants (RR, 0.90; 95%, CI 0.82-0.99) or children (RR, 0.82; 95% CI, 0.73-0.92),
respectively.

Because these data did not address brain cancers in the youngest individuals,
we examined US cancer mortality rates among individuals younger than 5 years.
The cohort exposed as children was not immunized with IPV until after their
fifth birthday. However, brain cancer mortality was higher in this group (2.04
per 100000 person-years) than in the cohort exposed as infants (1.27 per 100000
person-years). Brain cancer mortality, therefore, was greater among young
children not yet vaccinated than in young children injected with contaminated
IPV during infancy. Notably, in the cohort born after 1963 and never exposed
to SV40-contaminated vaccine, the rate was 1.04, showing that brain cancer
deaths among infants continued to decrease over time. Each rate was significantly
different from the others based on a total of 4643 brain cancer deaths and
333163427 person-years of observation.

Osteosarcomas were studied in the age groups at highest risk of developing
the disease, the teenage and young adult years (Figure 1, C). The age-specific incidence data, based on 522 total
cases in the data set, were well fit using a cubic spline with 2 segments
(goodness of fit, 94.87 on 75 df). However, Poisson
regression showed no significant differences in risk (χ2, 2.12
on 2 df;P=.346) between
the unexposed cohort and the cohorts exposed as infants (RR, 0.87; 95% CI,
0.71-1.06) or children (RR, 0.85; 95% CI, 0.59-1.22). Note the initial peak
at 10 years of age in the cohort exposed as infants represented just 2 cases
(95% CI, 0.55-16.53). Similarly, trends in osteosarcoma incidence rates over
time in Connecticut showed no increases that could be attributed to SV40-contaminated
vaccines (data not shown).

Mesotheliomas (Figure 1, D)
showed no significant cohort effect (χ2, 2.90 on 2 df;P =.23) in the linear age model that provided
the best fit (goodness of fit, 60.13 on 78 df). The
risk in the cohorts exposed as infants (RR, 3.00; 95% CI, 0.67-13.11) or children
(RR, 2.45; 95% CI, 0.50-12.03) was elevated as compared with the unexposed.
However, the birth cohorts studied have not yet reached the age at which most
mesotheliomas occur, resulting in few cases (a total of 71) and imprecise
estimates of risk.

Comment

Contamination of the early poliovirus vaccines with SV40 has reemerged
as a public health concern following recent reports that SV40 DNA may be present
in osteosarcomas, mesotheliomas, ependymomas, and perhaps other types of brain
cancer.3- 7
The extensive parenteral exposure of infants is a particular cause for concern
as animal studies have shown that injected neonates are particularly susceptible
to SV40-induced tumors.1,2,8,9,17
However, more than 30 years after this extensive single-source exposure in
the United States, the birth cohorts exposed as infants or children showed
no significant increase in those cancers reported to have high prevalence
of SV40 DNA.

This result is reassuring, as it is likely that we would have observed
an effect on cancer rates if one existed. As discussed, almost all US children
under the age of 20 years in 1961 had been injected 1 or more times with SV40-contaminated
IPV.8 Furthermore, because of the large number
of individuals studied and the long period of follow-up, each cohort contributed
a large number of person-years to the data. To help judge the uncertainty
in our analyses of incidence rates, we calculated the 95% CIs around the estimates
of cancer risk in the exposed birth cohorts. For ependymomas and osteosarcomas,
even the upper limit of risk was quite small, and for brain cancers there
was a significant inverse relation. Few cases of mesothelioma occurred in
any groups.

A causal relation between SV40 exposure and ependymomas in children
would involve a short incubation time, if the recent detection of SV40 DNA
in ependymomas in infants is to be believed. Therefore, the absence of an
SV40-contaminated vaccine effect on ependymoma cancer rates in the Connecticut
children 0 to 4 years of age is consistent with the cohort analyses. Together
the null results argue against a relation between vaccine-related SV40 exposure
and the development of ependymomas.

In addition, overall brain cancer incidence rates were actually lower
in the exposed birth cohorts. This pattern seems unlikely to represent a protective
effect of SV40-contaminated vaccines, but it probably reflects the increase
in brain cancer incidence over calendar time that has been well described
in the literature.20 To specifically evaluate
brain cancers in young children and infants we assessed cancer mortality rates,
but no relation was seen between SV40-contaminated vaccine exposure and the
development of brain cancers in children under 5 years of age.

The age-specific incidence of osteosarcoma was not significantly different
in exposed or unexposed cohorts, including the teenage years when osteosarcomas
are most common.21 In addition, trends in osteosarcoma
incidence in Connecticut showed no changes that could be attributed to the
period of vaccine contamination. The interpretation of this finding is limited,
since the postulated incubation time of SV40-induced osteosarcoma is not as
defined as it is for ependymoma. However, the overall patterns observed for
osteosarcoma incidence argue against an association with vaccine-related SV40
exposure.

Mesothelioma incidence rates showed a nonsignificant increase among
the exposed groups. Few individuals developed mesothelioma in any of the comparison
groups, however, and the modest case numbers made estimates of RR imprecise.
Mesotheliomas could not be directly studied in the older age groups, which
are ordinarily at highest risk, since individuals in the exposed cohorts were
at most 46 years of age in 1993. This is important, as mesothelioma incidence
has increased dramatically over time, but only among older individuals who
were unlikely to have received the contaminated vaccines. Therefore, other
factors, notably asbestos exposure, likely explain the increases in mesothelioma
incidence rates that have been observed. Final conclusions about the relation
of mesotheliomas to SV40-contaminated vaccines will not be possible until
the individuals exposed as infants and children reach a more advanced age.

Several limitations to this investigation need to be considered. It
is important that this report not be viewed as strong evidence against the
role of SV40 as a human pathogen. For example, SV40 may have been in the human
population for some time, unrelated to vaccine exposure, as suggested by the
finding of SV40 antibodies in serum samples around the world that were collected
before introduction of poliovirus vaccines.8
It is also possible that SV40 only has tumorigenic potential in humans exposed
under different conditions and higher levels of virus than were associated
with poliovirus vaccine. Vaccine-related exposure to SV40 in many countries
has involved either oral administration or mostly low viral titers in injected
inoculations.8 In general, the unavailability
of specific information regarding the actual SV40 titer of each inoculation
has limited the power of population-based studies of this kind. Finally, comparisons
among birth cohorts measure the net impact of all protective and adverse factors
that influence the risk of cancer in the cohorts, and not just the factor
under investigation (ie, SV40 exposure).

In summary, our study failed to detect any significant increases in
the risk of cancers reported to contain SV40 DNA among the birth cohorts exposed
to SV40-contaminated vaccine. In effect, ependymomas and osteosarcomas have
remained rare cancers,20,21 while
the rising rates for mesotheliomas have involved older age groups unlikely
to have received SV40-contaminated vaccine. Thus, approximately 30 years after
millions of Americans were parenterally exposed as infants or children, the
absence of a discernible effect in our study adds to the evidence that no
relation exists between exposure to SV40-contaminated vaccine and the development
of cancer. As the exposed cohorts mature, however, it will be important to
continue monitoring of cancer risks.